1. Field of the Invention
The present invention relates to a stencil printer for printing an image on a sheet by pressing the sheet against a master wrapped round a print drum with pressing means and a stencil printer capable of printing images on both surfaces of a sheet in a single step.
2. Description of the Prior Art
A digital, thermosensitive stencil printer is known as a relatively simple, convenient printer and uses a stencil made up of a thermoplastic resin film and a porous support adhered together. In the stencil printer, while a thermal head, provided with a number of heat generating devices, is held in contact with the stencil, a platen roller or similar conveying means conveys the stencil. At this instant, current is selectively fed to the heat generating devices in the form of pulses in order to perforate, or cut, the thermoplastic resin film of the stencil with heat in accordance with image data, thereby producing a master.
After the master thus produced has been wrapped round a porous print drum, a sheet is pressed against the print drum via the master by a press roller or similar pressing means with the result that ink, applied to the inner periphery of the print drum, is transferred to the sheet via the porous portion of the print drum and perforations formed in the master, printing an image on the sheet.
Generally, in the stencil printer described above, the press roller is rotatably supported by a pair of arm members constantly biased toward the print drum by biasing means. The arm members are caused to angularly move by, e.g., cams rotatable in synchronism with the rotation of the print drum, selectively moving the press roller into or out of contact with the print drum. To prevent the sheet being conveyed toward or away from the press roller from getting under the press roller, it is a common practice to locate a guide plate, frame included in sheet conveying means or similar member in the vicinity of the press roller upstream or downstream of the press roller in the direction of sheet conveyance.
The problem with the above stencil printer is that because a space available around the press roller is limited, it is difficult to replace the press roller when the press roller is worn out due to repeated operation or to remove a sheet when the sheet gets under the press roller due to some cause and brings about a jam.
In light of the above, Japanese Patent Laid-Open Publication No. 64-16355, for example, proposes a stencil printer including a press roller rotatably, removably supported by a pair of press roller arms which are, in turn, angularly movably supported by the printer body. The press roller arms are mounted on a shaft in such a manner as to be movable in the axial direction of the shaft and unlockable from an arm stay affixed to the above shaft, so that the press roller can be removed from the printer body.
The current trend in the imaging art is toward a duplex printing system configured to print images on both surfaces of a single sheet for, e.g., saving sheets and spaces for the storage of documents. Japanese Patent Laid-Open Publication No. 2005-125716, for example, discloses an example of a stencil printer capable of producing a print carrying images on both surfaces thereof, i.e., a duplex print in a single step. The single step, duplex printing type of stencil printer taught in this document uses a divided master in which a first and a second image are formed side by side in the direction of rotation of a print drum, and is constructed to print one of the two images on one surface of a first sheet fed from a sheet feeding section, guide the sheet to an auxiliary tray, print the above image on one surface of a second sheet also fed from the sheet feeding section, guide the second sheet to the auxiliary tray while again feeding, or refeeding, the first sheet from the auxiliary tray, print the other image on the reverse surface of the first sheet, discharge the resulting duplex print, and repeat such a procedure thereafter.
However, the prior art stencil printers stated above have some problems left unsolved, as will be described hereinafter.
The stencil printer taught in Laid-Open Publication No. 64-16355 has a drawback that a space available around the press roller is too small to efficiently mount or dismount the press roller. This makes the replacement of the press roller or the removal of a jamming sheet troublesome. This is also true with the stencil printer of Laid-Open Publication No. 2005-125716. Another problem with Laid-Open Publication No. 2005-125716 is that a refeed tray adapted to temporarily store a sheet carrying an image on one surface thereof, i.e., a simplex print is configured to angularly move integrally with the press roller, making the removal of a sheet jamming the refeed tray difficult to perform.
Further, a modern stencil printer is constructed such that a print drum is removable from the printer body in order to implement, e.g., color printing. In this case, while the print drum is generally pulled out from the printer body to the front along, e.g., rail members and then removed from the printer body, no members should preferably exist below the print drum to allow the print drum to move downward when being removed from the printer body. Should the print drum with an uncovered surface contact any other member at the time of removal, the uncovered surface would be scratched or otherwise damaged and would render printing defective.
The stencil printer disclosed in Laid-Open Publication No. 2005-125716 mentioned earlier includes refeeding means for temporarily storing a simplex sheet come out of a printing section and again feeding it toward the printing section. When the simplex print thus existing on the refeeding means is caused to overlap the next sheet without being refed due to some cause and jams the refeeding means or when a jam occurs with the edge of a sheet being left on the refeeding means due to the short conveying force of the refereeing means, it is difficult to remove the jamming sheet because the space available for such work is extremely limited.